1. Field of the Invention
The invention relates to an electrostatic chuck for attractively holding a wafer such as a semiconductor wafer thereon.
2. Prior Art
Conventionally, electrostatic chucks are used for accurately holding and positioning a semiconductor wafer (which hereinbelow will shortly be referred to as a “wafer”) thereon, primarily in a semiconductor manufacturing systems which include various deposition apparatuses (for example, for PVD, CVD, or plasma CVD processes) and etching apparatuses (for example, for plasma etching or optical etching).
Processes in many semiconductor-manufacturing apparatuses are often performed under a vacuum. In deposition apparatuses, reactive gas used in the process of deposition often heats a wafer on a chuck and causes the wafer to have a non-uniform temperature distribution on its surface, thereby in some cases, resulting in a deterioration in wafer qualities.
Also, wafers are heated by plasma gaseous etchant in etching apparatuses, or with ultraviolet or visible light illuminated in patterned photo-projection etching so that a temperature distribution over the wafer surface is made non-uniform, thereby causing variations in etching rate to an extent that the overall surface of the wafer cannot be uniformly etched. A problem of how to implement a uniform temperature distribution over the wafer surface has been held without resolution.
FIG. 11 shows a conventional electrostatic chuck disclosed in Japanese Patent Publication No. 07-153825. in which a conventional electrostatic chuck has a ceramic palate, an electrostatically attracting electrode 23 which is embedded in the ceramic plate 22, and many protrusions 24 which are provided over the upper surface of the ceramic plate 22. The chuck has an annular outer-peripheral portion 25, surrounding the protrusions 24, with the same level as that of the protrusion 24 and with a width 8 of 1 to 5 mm. According to the technique disclosed in the above publication, where a wafer W is mounted over the tops of the outer-peripheral portion 25 and of the protrusions 24, an electrostatic force is generated in a state where a voltage is applied between the wafer W and the electrode 23, thereby, attracting and fixedly supporting the wafer W onto the top faces of the protrusions 24 and the outer-peripheral convex portion 25. In etching or patterning treatment processes, heat-conductive gas such as helium is supplied into a space formed between the water W and the upper surface of the ceramic plate 22 to enhance heat conductivity between the wafer W and the electrostatic chuck and uniformize a temperature distribution on the overall surface of the wafer W.
In addition, FIG. 12 shows an electrostatic chuck 31 disclosed in Japanese Patent Publication No. 07-086385, in which, referring to the drawings, is provided with a metal plate 32 functioning as an electrostatic attraction electrode, and a recess portion 33 formed on an upper surface of the plate 32 except an outer peripheral portion 34. Further, a dielectric layer 35 is adhesively stacked over the upper surface including the recess portion 33 of the metal plate 32. The recess portion 33 has a depth r in a range of from several tens of micrometers to 0.1 mm or 0.2 mm. The depth r is a distance from a dielectric layer surface 35a on the top face of the outer peripheral portion 34 to a dielectric layer surface 35b on a bottom surface of the recess portion 33.
In use of the electrostatic chuck, a voltage is applied between the plate 32 and a wafer W whose peripheral portion is placed over the dielectric layer surface 35a on the top face of the outer peripheral portion 34. Thereby, only the peripheral portion of the wafer W is attracted and fixedly set onto the dielectric layer surface 35a on the top face of the outer peripheral portion 34. Similarly to the above, a heat-conductive gas is supplied into a space formed between the wafer W and the recess portion 33 of the plate to enhance heat conductivity between the wafer W and the electrostatic chuck 31, and the temperature distribution over-the wafer W is thereby intended to be made uniform.
However, the electrostatic chuck according to Japanese Patent Publication No. 07-153825 attractively holds the wafer W via the top surfaces of the many protrusions 24 together with an outer-peripheral portion 25 so that an wafer W with a flection or warp cannot perfectly attracted on the chuck while such a warped wafer is only partly attracted thereon, resulting in lack of attraction force to the wafer, thereby causing, for example, migrating of the wafer on the chuck, or in some cases, falling of the wafer from the chuck stage.
In this prior chuck, while the inner region in the outer-peripheral portion 25 is may be an attraction region, no electrostatic force is generated between the outer-peripheral portion 25 and the wafer W, since the electrostatic attraction electrode 23 buried has not reached a portion immediate beneath the outer-peripheral portion 25. Consequently, when a wafer W deformed with a flection or warp is mounted on the chuck, many gaps are developed between the wafer W and the top face of the outer-peripheral portion 25 and unexpectedly allow the heat-conductive gas to leak from the space under the wafer, which reduces a degree of vacuum in a semiconductor manufacturing apparatus, adversely affecting dimensional precision of deposited films and etched patterns formed in films.
Also, the electrostatic chuck 31 disclosed in Japanese Patent Publication No. 07-86385 poses problems as follows. In the electrostatic chuck 31, the large recess 33 is provided in the central portion, with only the stepped portion of the outer peripheral portion 34 around the recess portion 33 used to attractively hold the wafer W, and since the attraction force is small, a supply pressure of the heat conductive gas supplied into the space between the wafer W and the recess portion 33 causes a gap partly between an outside of the wafer W and a top face of the outer peripheral portion 34,. The resulting gap allows a leakage of the heat-conductive gas, thereby reducing a vacuum during processing and adversely affecting dimensional of fabricated films thereon
As described above, in the etching, a wafer is heated by a plasma etching gas, ultraviolet light, and visible light in photo-excited etching. As such, research has been conducted regarding how to cause heat exerted on the water to be escaped or transferred away.
To overcome these problems described above, a technique has been proposed that uses an electrostatic chuck to which a base member including a cooling mechanism is adhered.
FIG. 13 shows another conventional electrostatic chuck 51 including a wafer-holding member 52. The wafer-holding member 52 includes a disc-shaped ceramic plate 53 and an electrostatic attraction electrode 56. One main surface of the ceramic plate 53 is formed as a mounting surface 54 for mounting a wafer W, and the electrostatic attraction electrode 56 is formed on the other main surface 55 of the ceramic plate 53. In addition, the electrostatic chuck 51 includes a base member 57 adhered via an adhesive layer 59 onto a surface on an opposite side of the mounting surface 54 of the ceramic plate 53.
A base member 57 includes fluid paths 58 through which a cooling gas and a cooling water flows. Thereby, the base member 57 is cooled, and heat exerted on the wafer W attractively held to the wafer-holding member 52 is thereby absorbed.
In addition, FIG. 14 shows another electrostatic chuck 51. In the electrostatic chuck 51, one main surface of a disc-shaped ceramic plate 53 is formed as a mounting surface 54 for mounting a wafer W, and a base member 57 is adhered via an adhesive layer 59 on the side opposing a mounting surface 54 of a wafer-holding member 52 embedded the electrostatic attraction electrode 56 in the ceramic plate 53. Also in the electrostatic chuck 51, similar to the electrostatic chuck 51 shown in FIG. 13, a cooling gas and a cooling water are supplied to flow along fluid paths 58 formed in the base member 57. Thereby, the base member 57 is cooled, and heat exerted on the water W attractively held to the wafer-holding member 52 is dissipated.
In the individual wafer-holding members shown in FIGS. 13 and 14, the mounting surface of the planner ceramic body is prepared and finished to substantially the same surface roughness as that of the other main surface to prevent the wafer-holding member from warping (refer to Japanese Patent Publication No. 2001-168177).
However, as described above, the temperature on the side of the mounting surface 54 of the wafer-holding member 52 is increased by various processes, whereas the temperature of the surface on the opposite side of the mounting surface 54 of the wafer-holding member 52 has been decreased by the cooling mechanism of the base member 57. This causes warpage in the ceramic plate 53, thereby deforming a central portion of the ceramic plate 53 to be convex toward the mounting surface 54. Consequently, since the flatness of the mounting surface 54 is reduced at the time of various water processes, processing accuracy for the wafer W cannot be enhanced. In particular, the thinner the thickness of the ceramic plate 53 forming the wafer-holding member 52; the more conspicuous the warpage of the planner ceramic body appears.
In order to increase the attraction force of the wafer-holding member 52, the distance between the mounting surface 54 and the electrostatic attraction electrode 56 needs to be reduced as possible. In the example shown in FIG. 13, ordinarily, a ceramic plate 53 having a thickness in a range of from 0.5 to 3.0 mm was used.
In the example shown in FIG. 14, since the wafer-holding member 52 is formed such that the electrostatic attraction electrode 56 is embedded in the ceramic plate 53, a ceramic plate having a reduced thickness in a range of from 2.0 to 5.0 mm is used for the planar ceramic body 53 in order to dissipate the heat exerted on the wafer W attractively held on the mounting surface 54 easily. There arises a problem that thin ceramic plates having a thickness not greater than 5.0 mm are apt to be warped.